Sunnis and Shias differ in doctrine, ritual, law, theology and religious organisation. It is the largest and oldest division in the history of Islam.

But the origins of the split lie in a dispute over who should have succeeded the Prophet Muhammad as leader of the Muslim community when he died in 632.

One group of Muslims elected Abu Bakr as the next caliph (leader) of the community, but another group believed the prophet's son-in-law, Ali, was the rightful successor.

Shias re-enact the battle near Karbala in which Hussein was killed

Though Ali eventually became the fourth caliph, his legitimacy was disputed and he was murdered in 661.

The Shiat Ali ("Party of Ali") refused to recognise the legitimacy of his chief opponent and successor, Muawiya.

Ali's sons Hassan and Hussein continued to oppose Muawiya and his successor, Yazid, and fighting between the two sides resulted. Hassan was poisoned in 669 and Hussein was killed in battle near Karbala in 680.

The ascendancy of Iraq's Sunni community began under the Sunni Ottoman Turks, whose empire ruled the Middle East for nearly 400 years.
Ottoman defeat in World War I did not end Sunni dominance.

In the 1920 Mandate of Iraq, the British worked to check the Shia majority's power by keeping Sunni Arabs in senior positions in government and the armed forces.

The Sunni officers in the army became increasingly politicised and eventually overthrew the British-appointed monarchy in 1958.

The coup by the secular Arab Socialist Baath Party five years later did not redress the inequalities, as the Sunni Arab elite were unaffected.

Increasingly disenfranchised and concerned by the growth of secular parties supported by the government, Shias mobilised around prominent clerics and began to campaign for a return to Islamic principles in government and social justice.

In 1979, the Islamic Revolution in Iran - where Shias constitute 89% of the population - galvanised Shia opposition to the Baath Party and made Saddam Hussein, now president, increasingly fearful of a similar revolution in Iraq.

When Shia political activists attempted to assassinate the deputy prime minister in 1980, Saddam responded by executing Ayatollah Mohammed Baqir Sadr, the uncle of radical cleric Moqtada Sadr, the first time so senior a cleric had been killed.

When Iraq declared war on its predominantly Shia neighbour, Iran, Saddam's government intensified its brutal crackdown.

Thousands of Shia were expelled to Iran or imprisoned, tortured and killed. Religious practices were restricted and pilgrimages to holy shrines were curtailed.

In 1991, after the Gulf War, the US President George Bush encouraged Iraqis to rise up against their leader.

Lacking US support, the massive southern rebellion was swiftly and brutally suppressed.